The present invention relates to a process for dewatering fine coal. In one aspect, this invention relates to a process for filtering an aqueous slurry of fine coal. In another aspect, this invention relates to a process for drying fine coal.
In the mining, cleaning and transporting of coal, considerable quantities of water and fine coal are processed. For example, continuous coal mining, which uses large quantities of water, produces wet coal which is finer and dirtier than coal produced by other mining processes. Additionally, coal-cleaning frequently includes water-washing, crushing and froth-floating which produce slurried fine coal. Furthermore, transporting coal in water-slurry form is sometimes practiced.
Fine coal is dewatered for various reasons, such as to improve its storing, handling and burning. This dewatering of fine coal is currently done by centrifuging and/or filtering a slurry of fine coal and evaporating water from wet coal obtained thereby. Centrifuging or filtering an aqueous slurry can produce fine coal having a water content of approximately 15 to 20 percent by weight. Drying can remove up to 100 percent of the surface water of fine coal. For example, drying of fine coal at 90.degree. C. to constant weight provides fine coal having a surface water content of zero percent, although capillary water may still be present.
For the purposes of this disclosure, the term fine coal means coal that will pass through a 28 mesh screen; the term water content means the total amount of surface water in a coal sample, excluding capillary water in the coal; and the term dewatering is generic to separating, such as filtering or centrifuging, and drying, such as evaporating, heating or blowing.
Because thermal drying of fine coal requires considerable energy and adds to air pollution, the centrifuging and filtering of fine coal slurries have received extensive and intensive study by the industry with the objective of decreasing the water content of the coal to the extent that thermal drying can be reduced or eliminated. While the mechanical aspects of centrifuging and filtering a slurry of fine coal such as slurry throughput, slurry temperature, slurry concentration, air flow, pressure differential, centrifuge speed and disk speed have received considerable attention, the chemical aspects thereof have received only slight attention.
It has been known for a long time that the use of certain surfactants improve the dewatering of fine coal via filtering. For example, small amounts of sodium lauryl sulfate, mixed with a fine coal slurry to be filtered or, alternatively, sprayed as an aqueous solution on a filter cake of fine coal, produces a drier product than the product obtained by unmodified filtration. On the other hand, the use of lauryl ammonium bromide as the surfactant is substantially ineffective in the same experiment.
However, the use of surfactants is not completely satisfactory. For example, sodium lauryl sulfate can cause undesirable foaming of the slurry, and blinding of the filter during the filtering process.
It is also disclosed by Gordon et al. in an application for U.S. patent, assigned to the assignee of this invention and filed on even date therewith, that an aqueous slurry of fine coal may be dewatered, with unexpected results, by mixing a water-soluble organopolysiloxane or a water-emulsifiable organopolysiloxane with the aqueous slurry of fine coal. While the process of Gordon et al. improves the dewatering of fine coal, further improvements therein are desired.